Typically, a high lift loader, exemplified by Lull U.S. Pat. No. 4,147,263, has an elongated telescopic boom pivotally secured at one end to a vehicle, and a carriage such as a fork lift pivotally secured to the opposite end of the boom. An hydraulic cylinder is used to raise and lower the boom relative to the vehicle and another hydraulic cylinder is used to tilt the carriage relative to the boom. Such vehicles are in common use for building construction and similar purposes.
Because the attitude of the carriage changes as the boom is raised or lowered on its pivot connection to the vehicle, constant adjustment of the flow of fluid to the hydraulic cylinder operating the carriage is necessary to maintain the desired level load carrying position of the carriage. Current methods for maintaining the desired relative positioning of the boom and carriage include mechanical parallelogram systems or mechanical-master/hydraulic-slave loop systems. However, use of these systems is not always possible nor practical.
Lull U.S. Pat. No. 589,950 is directed to a load self-leveling system for a vehicle utilizing hydraulic cylinders to operate a telescopic lifting boom and a material handling carriage in which the system includes a pair of fluid flow dividers interposed between the source of hydraulic fluid under pressure and the hydraulic cylinders operating the boom and carriage. The flow dividers are structured to direct a greater flow of hydraulic fluid to the boom cylinder and a lesser flow to the carriage cylinder, the precise numeric proportions of the flow division being engineered to best suit a particular vehicle, combined with the specific componentry of that vehicle. In addition to the flow dividers, the system includes reversible flow piloted check valves, override control including override relief valves to allow independent adjustments, and matched cylinder area ratios.
While the self-leveling system of the Lull patent is highly useful and a significant improvement over prior systems, when different size material handling units are built, the load leveling system, and in particular the spool valve plugs, for each different unit must be sized accordingly. Thus, from a manufacturing standpoint, a number of valve plugs for such load leveling systems must be made available and maintained in stock. Furthermore, as the load leveling system operates, the parts wear due to the shuttling action of the spool valve plugs. Over a long period of time, this wear can cause a portion of the hydraulic fluid to bypass the plug, thus reducing the efficiency of the flow divider system and necessitating the repair or replacement of the flow divider system. Finally, while the spool valves may be adjusted at the factory for proper operation, there is no simple mechanism for "fine tuning" the system in the field to obtain optimum performance after its installation on the vehicle. If minor adjustments are needed during use of the vehicle, the system must be removed so that the spool valve plugs can be repaired or replaced.
The object of the present invention, therefore, is to provide an improvement upon the system described in the Lull '802 patent application which resolves all of the aforementioned deficiencies in a universally applicable system for automatically maintaining any desired attitude of the material handling carriage attachment relative to the lifting or hoisting boom. In addition, if desired, an override feature can be included to allow independent adjustment of the system.